This invention relates generally to an imaging light detection and ranging (lidar) system. More particularly, this invention relates to a lidar system which is adapted to accomplish variable time delay range gating across a selected image.
An imaging light detection and ranging (LIDAR) system is generally depicted in FIG. 1. The LIDAR system of FIG. 1 is of the type described in U.S. Patent Application Serial Nos. 216,341 filed July 7, 1988, now U.S. Pat. No. 4,862,275 and 256,778 filed Oct. 12, 1988, both of which are assigned to the assignee hereof and incorporated herein by reference.
In the Lidar system of FIG. 1, a pulsed light source 10 (such as a pulsed laser) emits a brief intense illuminating pulse of light 12 toward a backscattering medium 13 upon receiving a signal from timing electronics 14 (e.g. timing generator). After a time delay corresponding to the round trip propagation time t=2Rn/c (where n is the index of refraction of the propagation medium, R is the target range and c is the velocity of light propagation in a vacuum); a second timing signal is sent from timing generator 14 to a range-gated camera 16. Upon receipt of this signal, camera 16 opens its shutter for a brief period .delta.t=(2n/C) (.delta.R) seconds, whereby reflection images of all targets 17 within the angular field of view 18 of camera 16 and within the range gate .delta.R are formed at the camera output.
As can be seen in FIG. 1, the surface of constant time delay is not planar, but is actually a sphere, a section of which is shown in the two-dimensional FIG. 1 as an arc of a circle 20.
However, for some applications, it is desired to image a plane (as opposed to a sphere) in target space, as shown by the dashed line 22 in FIG. 1. In current lidar systems, the time delay is constant over the entire field of view, resulting in the imaging of only an annulus 24 at target image plane 22. The light from the target areas inside and outside this annulus are received either before (inside area identified at 26) or after (e.g., outside areas identified at 28) the image from range R. Therefore, a large part of target plane 22 is not imaged by camera 16 in a single picture. Moreover, the picture which is imaged will vary in intensity for different values of range R if the propagation medium is not perfectly transparent.